1. Technical Field
The present invention relates generally to CATV program security systems, and more particularly to television scrambling systems employing SAW notch filters.
2. Background Art
There is known a scrambling system described in U.S. Pat. No. 4,074,311 to Tanner et al., wherein the scrambling process is performed by inserting a jamming carrier into the TV signal spectrum between the audio and video carriers, on the transmit or headend side, and descrambling is implemented by means of a notch filter which removes the jamming carrier on the receive side.
The Tanner system has a number of drawbacks. First, although the central spectral components of the scrambled TV signal are pre-amplified on the transmit side, a significant portion of the TV signal spectrum is removed during descrambling on the receive side. This drawback causes blurring and is highly perceptible when transmitting text which spectrum includes RF components, as well as when transmitting a scrambled signal at high frequencies as that results in a broadening of the notch in the descrambling filter.
Another disadvantage of the Tanner system is that a jamming carrier is offset far from the picture carrier in frequency, and the notch filter is realized using LC-components, both aspects of which make the system relatively easy to defeat by unauthorized subscribers.
Further, the Tanner system cannot be practically implemented in a coded jamming system, as proposed in U.S. Pat. No. 4,623,918 to Chomet, because the notch filter in Tanner removes too much of the TV signal spectrum when removing the jamming carrier. A coded jamming system involves placing more than one jamming carrier in the TV signal spectrum, or placing a single jamming carrier at one of a plurality of assigned locations in the TV signal spectrum. A coded system is theoretically more secure than the system proposed in Tanner et al., but, heretofore, has not been realizable in a practical and economical scrambling system.
There is also known a scrambling system proposed in U.S. Pat. No. Re. 34,720 to Zelenz, wherein the TV signal spectrum is distorted by a filter whose amplitude-frequency characteristic is shaped like a Gaussian curve. This system offers significant improvement in TV picture quality over the Tanner et al. system; however, the system is not designed for a coded jamming implementation.
There is a scrambling system introduced in U.S. Pat. No. 4,748,667 to Farmer et. al. and U.S. Pat. No. 5,068,893 to West et al., wherein a jamming signal is placed in the vestigial sideband (VSB) of the TV signal and a SAW notch filter is employed to remove it. Distortions introduced by the SAW notch filter are compensated by suppression of the TV signal spectrum in the vestigial sideband (VSB) and amplification of the spectrum in the main sideband (MSB). The jamming signal is placed very close to the picture carrier in the VSB, and the narrow-band SAW notch filter removes it upon descrambling.
In theory, the Farmer/West system offers greater program security than the Tanner system. However, for efficient scrambling, the jamming signal should be offset from the picture carrier no more than 300 kHz in the VSB, otherwise it will be suppressed by the TV receiver. Within the 300 kHz offset, the SAW notch filter introduces phase distortions into TV signal. The compensation technique proposed in Farmer et al. and West is only suitable for compensating distortion due to the low frequency branch of the SAW filter's group delay characteristic. The distortion due to the high-frequency branch of the group delay, where the picture carrier and nearby TV signal spectrum fall, is not compensated by this technique. This results in low-frequency distortions to the TV picture after descrambling with the SAW notch filter. Furthermore, this proposed system is not suitable for a coded jamming implementation.
There is a scrambling system proposed in U.S. Pat. No. 4,903,297 to Rist et al., wherein the scrambling process is executed by means of placing a jamming signal into both the MSB VSB, and applying a SAW notch filter for descrambling. On the transmit side, the TV signal distortions introduced by the SAW notch filter are compensated by amplitude and phase pre-correction of the TV signal. Pre-correction is accomplished by means of amplifier, or by serial amplifiers, with negative feedback containing the same SAW notch filter applied for descrambling on the receive side.
Despite pre-correction of phase and amplitude in the Rist system, optimum TV picture quality is not achieved in practice because of incomplete compensation of the phase and amplitude distortions introduced by the SAW notch filter. Phase and amplitude compensation is incomplete because ideal pre-correction (i.e., the inverse of the amplitude and phase characteristics of the SAW notch filter) can only be achieved with an infinitely large gain factor of the amplifier, enveloped by negative feedback. The slope of the phase-frequency characteristic of the SAW notch filter is very steep within the notch frequency region; thus, when such a filter is switched into the negative feedback circuit of the amplifier, the latter will go into self-oscillation. Stable operation is only assured when the gain factor is small, but that results in less than ideal pre-correction of phase and amplitude.